That seems hardly likely. The Law was what guided creation. The 7th day was created and "rested" by God Himself when creation was finished. He ordained it "in the beginning". Adam kept it as did his progeny though it is not recorded.
Plainly Wrong. The everlasting covenant of circumcision was instituted in Gn. 17, long before the giving of the Law. Nor did God manifest observing the 7th day as part of what constituted obedience to God before the Law, while things such as fornication/adultery, lying, envy, murder, stealing, etc. and even eating blood are all manifest as contrary to what God required of man long before the giving of the Law.
Adam kept it as did his progeny though it is not recorded.
Such a bold assertion is reading into Scripture what you can only wish was there. Do you really think the Holy Spirit would leave such a primary requirement and or practice out of the examples of men before the Law?
If 7th day worshipers wrote the Bible then the requirement or custom of keeping the 7th day would be right there with the command to procreate.Instead, the 7th day worshipers cannot claim Abraham as an example, nor anyone else before the Law.